Membrane Structure and Function

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Membrane Structure and Function

• Chapter 5

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Outline

• Membrane Models
• Fluid-Mosaic
• Plasma Membrane Structure and Function
• Protein Functions
• Plasma Membrane Permeability
• Diffusion
• Osmosis
• Transport Via Carrier Proteins
• Cell Surface Modifications

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Membrane Models

• Robertson- Unit membrane
• Singer and Nicolson - Fluid-Mosaic Model
• Membrane is a fluid phospholipid bilayer in which
  protein molecules are either partially or wholly
  embedded.

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Membrane Models
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Plasma membrane structure

• Phospholipid bilayer- hydrophobic (tails) fatty
  acids face inward, hydrophilic (head) phosphate
  group faces toward the outside of cell and
  towards cytoplasm on the inside layer.
• Some lipids have sugar portions added to them-
  glycolipids

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Fluid-Mosaic Model
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Plasma Membrane Structure and Function

• Plasma membrane separates internal environment
  from the external environment.
• Hydrophilic polar heads face outside, and
  hydrophobic non-polar tails face each other.

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Cholesterol

• Is found in the inside of the cell membrane in
  the hydrophobic portion
• The more cholesterol there is in a membrane the
  less permeable the membrane (inverse relationship)

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Experiment to demonstrate lateral movement of
proteins

• Tagged membrane receptors move in the membrane at
  about 2?m per second
• When two cells (which have different receptor
  proteins) are fused
• The receptors move and become evenly dispersed

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Membranes are fluid

• The higher the concentration of unsaturated fats
  the more fluid the membrane is
• Fluidity of membrane structure helps maintain a
  pliable (flexible) membrane very important for
  example red blood cells

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Plasma Membrane Structure and Function

• Proteins may be peripheral or integral.
• Peripheral proteins are found on the inner
  membrane surface.
• Integral proteins are embedded in the membrane.

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Protein Functions

• Channel Proteins - Involved in passage of
  molecules through membrane.
• Carrier Proteins - Combine with substance to aid
  in passage through membrane.
• Cell Recognition Proteins - Help body recognize
  foreign substances.

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Protein Functions

• Receptor Proteins - Allow molecule binding,
  causing protein to change shape and bring about
  cellular change.
• Enzymatic Proteins - Carry out metabolic
  reactions directly.

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Passage of molecules into and out of the cell

• Passive processes are driven by kinetic energy of
  molecules not cellular energy
• Active processes are driven by ATP (cellular
  energy)

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Membrane permeability

• Membranes are selectively permeable
  (semipermeable and differentially permeable)
• Some molecules freely enter and leave a cell,
  others have to go through membrane channels
• Large objects such as bacteria, viruses, are
  engulfed

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Plasma Membrane Permeability

• Plasma membrane is differentially permeable.
• Passive Transport - No ATP requirement.
• Molecules follow concentration gradient.
• Active Transport - Requires carrier protein and
  ATP.

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Passive processes

• Diffusion
• Osmosis
• Facilitated transport

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Crossing Plasma Membrane
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Diffusion

• Diffusion - Movement of molecules from a higher
  to a lower concentration until equilibrium is
  reached.
• Down concentration gradient
• A solution contains a solute (solid) and a
  solvent (liquid).

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Diffusion

• Solute molecules diffuse from an area of higher
  concentration to an area of lesser concentration
• Examples- when you dye a cell, the dye molecules
  move from an area of higher concentration to a
  lower even across a cell membrane if the dye
  molecules are small enough

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Diffusion
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Gas exchange in the lungs

• Oxygen moves from an area of higher concentration
  (lungs) to an area of lower concentration (blood
  vessels)

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Osmosis

• Osmosis - Diffusion of water across a
  differentially (selectively) permeable membrane
  due to concentration differences.
• Osmotic pressure is the pressure that develops
  due to osmosis.
• The greater the osmotic pressure, the more likely
  water will diffuse in that direction.

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Osmosis

• Water moves across a semipermeable membrane from
  an area of higher water concentration to an area
  of lower water concentration

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Osmotic pressure

• Is due to solute concentration
• The osmotic pressure is higher in the compartment
  which has the higher solute concentration.
• The greater the gradient (higher osmotic
  pressure) the more water moves
• If the osmotic pressure is higher inside the
  cell, water is drawn into the cell

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Osmosis in animal and plant cells

• Isotonic condition
• Hypotonic condition
• Hypertonic condition

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Osmosis

• Isotonic Solution - Solute and water
  concentrations both inside and outside the
  membrane are equal.
• Hypotonic Solution - Solution with a lower
  concentration of solute than the solution on the
  other side of the membrane.
• Cells placed in a hypotonic solution will swell.
• Lysis

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Osmosis

• Animal cells
• Plant cells

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In animal cells- hypotonic

• Water moves into the animal cell. The cell gets
  larger and may burst
• .01 NaCl or distilled water would be considered
  hypotonic

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Hypertonic in animal cells

• The outside solution has MORE solute molecule and
  LESS water molecules
• Water moves out of the animal cell and the cell
  shrinks and crenation occurs
• A 10 NaCl solution is hypertonic for red blood
  cells

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Isotonic

• Animal cells stay the same size and
• the central vacuole of plants stay the same size
• For red blood cells a 0.9 NaCl (saline) solution
  is isotonic

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Isotonic

• Isotonic solutions- has the same solute
  concentrations and solvent concentration on the
  inside of the cell and the outside of the cell
• Water moves into and out of the cell at a equal
  rate so there is no Net movement of water

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Hypertonic in plants

• In plants, water moves out of the central vacuole
  and out of the cell. The central vacuole shrinks,
  and the plasma membrane pulls away from the
  inside of the cell wall, chloroplasts are seen in
  the center of the cell. Plasmolysis has occurred

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Hypotonic in plants

• Water moves into the plant cell central vacuoles
  and creates turgor pressure. The cell does not
  get larger, but the chloroplasts can be seen
  pushed against the inside of the cell wall and
  plasma membrane.

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Transport by Carrier Proteins

• Carrier proteins combine with a certain molecules
  which are then transported through the membrane.
• Facilitated Transport
• Small molecules follow concentration gradient by
  combining with carrier proteins.

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Facilitated transport

• Is a passive process
• Molecules are moving from an area of higher
  concentration to an area of lower concentration

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Facilitated transport

• Large molecules such as glucose can not diffuse
  across membranes, different cells have different
  glucose needs.
• Carrier proteins change shape as the molecule
  passes through the central portions of the protein

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Active transport

• Uses ATP
• Membrane pumps
• Exocytosis
• Secretion
• Excretion
• Endocytosis
• Phagocytosis
• Pinocytosis
• Receptor- mediated endocytosis

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Na/K pump

• Receptor in the membrane contains two channels
• Three sodiums move out of the cell
• ATP is used a change in the shape of receptor
• Two potassiums move into the cell

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Membrane-Assisted Transport

• Large marcomolecules are transported into or out
  of the cell by vesicle formation.
• Exocytosis - Vesicles fuse with plasma membrane
  as secretion occurs.

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Exocytosis

• Process which 'export' material out of the cell
• Secretion- macromolecules are useful to the
  organisms
• Example secretion of hormones
• Excretion- molecules which are released to the
  outside are waste molecules and or toxins

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Membrane-Assisted Transport

• Endocytosis - Cells take in substances by vesicle
  formation.
• Phagocytosis - Large, solid material.
• Pinocytosis - Liquid or small, solid particles.
• Receptor-Mediated - Specific form of pinocytosis
  using a coated pit.

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Phagocytosis

• Phagocytosis (cell eating) is when cells engulf
  particles such as bacteria
• Phagocytosis of internal parts also occurs, for
  example old or damaged mitochondria are engulfed

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Pinocytosis

• Also called 'cell drinking'
• Small vesicles are pinched inward which contains
  water and dissolved materials

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Receptor mediated endocytosis

• Specific molecules (ligands) bind to membrane
  receptors
• When receptors are full, the membrane pinches
  inward
• Example hormones (protein)
• Note Clathrin

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Plant cells have a cell wall

• And a plasma membrane
• Primary cell wall contains cellulose
• Pectins (polysaccharide) allow the cell wall to
  stretch when a plant is growing

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Plant cell walls

• Is extracellular and glue like
• Contains a high amount of pectin
• Connects two adjacent plant cells
• Plasmodesmata
• Strands of cytoplasm that run in channels
  connecting two plant cell

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Multicellular animal cells

• Cell surface modifications
• Cells have an extracellular matrix
• Collagen fibers (protein) gives strength to
  tissues
• Elastin fibers (protein) gives flexibility to
  tissues

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Function of proteins on the outside of cells

• Communicate with cytoskeleton (proteins) on the
  inside of cells
• Fibronectins and laminins are adhesive proteins
• Attach to membrane receptors on one end and to
  extracellular matrix on the other end

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Animal cell junctions

• Desmosomes
• Tight junctions
• Gap junctions

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Adhesion junction (desmosomes)

• Have cytoplasmic plague on cytoplasmic sides
• Are held together with intermediate filaments
  that weave between the cells
• Are found in hear, stomach, bladder where a lot
  of stretching is occurring

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Tight junction , Gap junctions

• Tight Junctions
• Attachment is 'zipper like found in the kidney
• Gap junction
• Allows cells to communicate
• Pores go through both membranes
• Found in hearts, ions flow rapidly from on cell
  to the next